Seasonal programming, not competition or testosterone, drives stress-axis changes in a partially-semelparous mammal

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Abstract

Animals must make tradeoffs between reproduction and longevity. This is particularly pronounced in male arctic ground squirrels (Urocitellus parryii), that compete aggressively for territories and mates during a three-week breeding season. Breeding males have high rates of severe wounding, high mortality rates, and high free cortisol levels, along with downstream consequences of chronic stress (weight loss, reduced immune function) that appear to contribute to their early death. The elevated cortisol levels are thought to be a result of the intense intrasexual competition. An alternative hypothesis, however, is that the hormonal change is a seasonal adaptation facilitating the tradeoff of immediate competitive advantage at the expense of long-term survival. We tested a two-part hypothesis: first, that elevated free cortisol during the breeding period is a seasonal change that will still occur in the absence of actual competition, and second, that testosterone maintains this increase. We measured plasma cortisol, corticosteroid-binding globulin, and fecal glucocorticoid metabolites in three groups: wild male ground squirrels, captive males prevented from fighting, and captive castrated males. There were no differences amongst these three groups in free and total plasma cortisol, fecal glucocorticoids, or downstream measures of chronic stress. This suggests that high free cortisol and its effects on breeding males are not a consequence of contest competition during the breeding season, but rather a generalized seasonal change. We found no evidence that testosterone plays a role in maintaining elevated free cortisol in arctic ground squirrel males.

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